State of the art engine technology may allow a diesel engine to emit as low as 0.6 gm/mile particulates. However, with more strigent particulate emission requirements to come into effect in 1985, such as at a level of 0.20 gm/mile, the technology cannot meet such lower level of particulate emissions without some form of particulate trap. The most important materials used to date by the prior art for the trap material have included rigid and fibrous ceramic filter materials (see U.S. Pat. No. 4,276,071, ceramic wall flow monolith particulate filter) and wire mesh (see U.S. Pat. No. 3,499,269), each material having its own characteristic mode of trapping. Some of these filter materials have been coated with catalysts in the hope it would facilitate incineration of the collected carbon material. Unfortunately, the placement of the coating as a layer throughout the filter has proven not to lower the incineration temperature effectively and, more importantly, has produced unwanted sulphates.
The particulates emitted and trapped throughout the life of a vehicle cannot be stored since the amount can be typically 20 cubic feet for each 100,000 miles of engine use. As the particulates build up, the exhaust system restriction is increased. Thus, a means is required to remove the trapped material periodically, commonly referred to as regeneration of the filter. One of the most promising methods found to date is rejuvenation of the filter by thermal oxidation of the carbonaceous particles, which incinerate at about 1200.degree. F. (600.degree. C.).
Normal diesel engine exhaust temperatures rarely reach 1200.degree. F. during normal driving. Therefore, an auxilliary temperature elevating means is necessary to carry out therma oxidation. The types of thermal oxidation means used by the prior art have generally fallen into the following three categories: use of a fuel fed burner (see U.S. Pat No. 4,167,852 and Japanese Pat. No. 55-19934), use of an electric heater (see U.S. Pat. Nos. 4,270,936; 4,276,066; 4,319,896), and detuning techniques, which may be combined with any of the above, for raising the temperature of the exhaust gas temperature at selected times (see U.S. Pat. Nos. 4,211,075; 3,499,269). These techniques have been used to burn the collected particles in the presence of excess oxygen.
With respect to fuel burners, they are disadvantageous because: (a) they require more fuel than for normal vehicle operation when they function as the sole means to raise the temperature of the entire mass of the filter system, and (b) can only be used for regeneration in certain limited cruise conditions of the engine when used in line with exhaust flow. In addition, such an addition is prone to malfunction and can pose safety problems unless an adequate control system is provided.
With respect to electrical heating used as the sole means to raise the temperature of the entire mass of the filter system to an incineration temperature: (a) it is inefficient; and (b) it requires a disproportionate supply of electrical power, which is not readily available with current vehicles, and would require significant redesign of the power supply system.
As to detuning techniques, they are difficult to operate to reliably achieve adequate incineration temperatures, may have an adverse effect on engine emissions, and may cause premature failure of the filter material.
What is needed is a filtration system for diesel engines which uses considerably less energy than that envisioned by the prior art regneration techniques, has increased reliability for incineration, does not affect other measures taken to control engine emissions, reduces the complexity of the controls needed for the regeneration system, and is independent of engine operation for optimum regeneration.